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2100 degree mold

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Ron Behrens
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2100 degree mold

Postby Ron Behrens » Tue Jul 30, 2013 6:11 pm

Can someone give some advise on glass molds good for casting to 2100 degrees. The plaster / silica molds just don't cut it. I am working on telescope mirror blanks How about kiln shelf material?

Joe Pfeifer
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Re: 2100 degree mold

Postby Joe Pfeifer » Tue Jul 30, 2013 7:09 pm

You might contact these folks. In the photos that play, you will see some pictures of a large telescope lens blank they made on a forklift:

http://glassfirestudio.com/

Bert Weiss
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Re: 2100 degree mold

Postby Bert Weiss » Tue Jul 30, 2013 11:20 pm

Unifrax Duraboards are rated to 2300ºF. This rating is all about how much the material will shrink at a temperature. These boards can be CNC carved and they can be used as a shelf. I believe Zircar mold mix 6 is good in this range.

The problem is that glass at 2100ºF is very corrosive and might not play well with any mold material. You can pour 2100º glass in to a bronze mold or a graphite mold, let it chill to the point where it won't distort, and then load it in to an annealer.

The pictures in Joe's link are of a countertop, not a lens, and I'm pretty sure this was a fuse in the high 1400's range.
Bert

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Joe Pfeifer
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Re: 2100 degree mold

Postby Joe Pfeifer » Tue Jul 30, 2013 11:24 pm

If you go to the website I mentioned, and click on the studio link, it shows other photos. They made a very large boro lens blank. shows the kiln they made it in too. I also know of a guy with a defunct ceramic lab that has some very large very high temp molyD bell style kilns for sintering ceramics. the temps go way higher, like 3000 F! probably get a $150K kiln for 10 cents on the dollar.

Ron Behrens
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Re: 2100 degree mold

Postby Ron Behrens » Wed Jul 31, 2013 12:21 pm

Bert
I have some unifrax fiberfrax. Is that the same? And is it "wetable" to where it can be molded onto a round shape to contain a telescope mirror?

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Re: 2100 degree mold

Postby Bert Weiss » Wed Jul 31, 2013 12:48 pm

Ron Behrens wrote:Bert
I have some unifrax fiberfrax. Is that the same? And is it "wetable" to where it can be molded onto a round shape to contain a telescope mirror?
Sort of. Maybe. Not exactly...

Fiber blanket can be rigidized to make molds. There are 2 kinds of rigidizers, colloidal silica and colloidal alumina. Fiber boards are made by rigidizing the fibers using a vacuum forming process, followed by heat. The boards can be machined, and then further rigidized.

Colloidal silica is the stronger rigidizer, but the silica sticks to glass. Alumina is a separator.

The catch to all this is how it behaves at temperature. I have plenty of experience working around 1500ºF. This is a good place to work. Once you get much hotter, everything becomes much more prone to sticking. I powder my fiber blankets and boards with finely ground alumina hydrate, and sticking is not a problem for me below 1500. I can't speak for above that.

I don't really understand the design requirements for the glass to make the tele mirror.
Bert



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Morganica
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Re: 2100 degree mold

Postby Morganica » Wed Jul 31, 2013 2:31 pm

You're right, typical plaster/silica isn't going to hold up long with that treatment. You can use a mullite/ceramic mold, but the separator will be a problem. Line it with at least quarter-inch of fiber paper and you should be OK. I wouldn't guarantee a long life for the mold.

Fiber will work, but again, it's going to break down much faster at those temps. I'd call the Ransom & Randolph folks and talk to them about ceramic slurry molds, the type used in bronze casting. I've used a type of those, and know a couple at least one glass casting expert who's done some amazing things with them. They do a great job, and they're very helpful.

Big question though, is why you need to go to 2100F for a soda-lime glass casting?
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Ron Behrens
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Re: 2100 degree mold

Postby Ron Behrens » Wed Jul 31, 2013 2:48 pm

Morganica wrote:You're right, typical plaster/silica isn't going to hold up long with that treatment. You can use a mullite/ceramic mold, but the separator will be a problem. Line it with at least quarter-inch of fiber paper and you should be OK. I wouldn't guarantee a long life for the mold.

Fiber will work, but again, it's going to break down much faster at those temps. I'd call the Ransom & Randolph folks and talk to them about ceramic slurry molds, the type used in bronze casting. I've used a type of those, and know a couple at least one glass casting expert who's done some amazing things with them. They do a great job, and they're very helpful.

Big question though, is why you need to go to 2100F for a soda-lime glass casting?


Morganica
I still have a lot to learn here. I have tried temperatures from 1400- 1800 with generally poor results. So I decided to try hotter and got better results I have not tried anything between 1800 and 2100. I tried a piece that I made and considered junk and at 2100 it turned into a useable piece. I do try to stay with similar glass when I do I am open to anything you guys can teach me. There is no one around near me to guide me

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Re: 2100 degree mold

Postby Morganica » Wed Jul 31, 2013 5:45 pm

Happy to help. ;-)

If you've got float glass, yes, it's not going to fully fuse together as well as glasses made especially for the purpose, and you'll probably have to get it hotter (or stay at lower temperatures much longer, which promotes devit).

And I know you said something about the thickness of the float glass, but how big in total are the pieces you're using? Can you fill your mold with a single piece? If you're mixing multiple pieces of glass in the same mold, you may get a fast lesson in incompatibility. Different glasses expand and contract at different rates, and if they're fused together in a mold, they may pull away from each other during cooling. That can result in a cracked piece when you open the kiln or, more insidiously, sometime later when the finished glass is exposed to thermal/mechanical stress. (The stress will show up on a polarizer)

That's a big part of the reason that fusing/casting/blowing glasses are more expensive than float--they're calibrated to remain compatible when combined. Generally speaking, if you've got "anonymous" float glass, i.e., you don't know its origin (or ideally, its origin and batch), you assume incompatibility between pieces and you don't put them together. In practice, I've had a compatibility issue with about 1 in 10 pieces. if the glass is free and you're up for making multiples, that's not a huge deal--you just accept a lower yield.
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Mike Jordan
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Re: 2100 degree mold

Postby Mike Jordan » Wed Jul 31, 2013 6:09 pm

I don't know a lot about creating lenses or mirror blanks but you have said you are making a mirror. Since you are going to have to grind to get it to the proper shape and then you are going to coat it with a reflective material, I wouldn't think it would matter what glass you used as long as it was all compatible. If it divits, you are going to be grinding that off during your shaping process. I wouldn't think you would need (or want) a solid blank either. You could make a mold with that has a lot of protrusions on it to take up space, which would mean you use less glass and be lighter weight. If I was trying to do this I'd probably buy cullet which, though it isn't really designed for fusing, would probably work better than float glass. Float glass would be cheaper though if you can pick up some big store window glass or a glass door.

A couple of years ago I read a blog of a guy in Australia (or maybe it was New Zealand?) that did his own blanks. He was pretty detailed in how it did it and outlined some of the problems he had to over come. I forget how long he had to anneal some of his glass but it was for a very long time. Kind of like start the anneal and go on vacation long. :D

Anyway, it just seems if it is just mirrors you are making and they are going to be ground and then coated, I would think that some of the considerations of the process would not be as important as they would be if you were doing art or something else like that.

Mike
It's said that inside each of us is an artist trying to get out. Well mine got out... and I haven't seen him since.

Ron Behrens
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Re: 2100 degree mold

Postby Ron Behrens » Wed Jul 31, 2013 6:26 pm

Yes I generally use material from a single piece of glass. I do however wonder about breaking up different glasses into tiny pieces and what the compatibility will be like in a full melt?

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Re: 2100 degree mold

Postby Peter Angel » Wed Jul 31, 2013 6:44 pm

I have cast into bisque fired earthenware paperclay without kilnwash at 1560F/850C and it's worked perfectly, but that will only work once (if you cast the 2nd time, you need kilnwash).

There are stoneware paperclays and porcelain paperclays that can take much higher firings. I think porcelain paperclay fires up to 2370F/1300C
Peter Angel
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Ron Behrens
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Re: 2100 degree mold

Postby Ron Behrens » Thu Aug 01, 2013 7:37 am

Bert Weiss wrote:
Ron Behrens wrote:Bert
I have some unifrax fiberfrax. Is that the same? And is it "wetable" to where it can be molded onto a round shape to contain a telescope mirror?
Sort of. Maybe. Not exactly...

Fiber blanket can be rigidized to make molds. There are 2 kinds of rigidizers, colloidal silica and colloidal alumina. Fiber boards are made by rigidizing the fibers using a vacuum forming process, followed by heat. The boards can be machined, and then further rigidized.

Colloidal silica is the stronger rigidizer, but the silica sticks to glass. Alumina is a separator.

The catch to all this is how it behaves at temperature. I have plenty of experience working around 1500ºF. This is a good place to work. Once you get much hotter, everything becomes much more prone to sticking. I powder my fiber blankets and boards with finely ground alumina hydrate, and sticking is not a problem for me below 1500. I can't speak for above that.

I don't really understand the design requirements for the glass to make the tele mirror.


Bert
I don't need a really perfect mold for the glass as I will machine it round when cool. I put it on a rotating table and while it spins a diamond grinder mounted on a movable vise grinds it true. Perhaps there are better ways but that is what I know know. I just need a mold that will hold it reasonably round. It sounds like artists need a lot more control than I do

Ron Behrens
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Re: 2100 degree mold

Postby Ron Behrens » Thu Aug 01, 2013 7:51 am

Mike Jordan wrote:I don't know a lot about creating lenses or mirror blanks but you have said you are making a mirror. Since you are going to have to grind to get it to the proper shape and then you are going to coat it with a reflective material, I wouldn't think it would matter what glass you used as long as it was all compatible. If it divits, you are going to be grinding that off during your shaping process. I wouldn't think you would need (or want) a solid blank either. You could make a mold with that has a lot of protrusions on it to take up space, which would mean you use less glass and be lighter weight. If I was trying to do this I'd probably buy cullet which, though it isn't really designed for fusing, would probably work better than float glass. Float glass would be cheaper though if you can pick up some big store window glass or a glass door.

A couple of years ago I read a blog of a guy in Australia (or maybe it was New Zealand?) that did his own blanks. He was pretty detailed in how it did it and outlined some of the problems he had to over come. I forget how long he had to anneal some of his glass but it was for a very long time. Kind of like start the anneal and go on vacation long. :D

Anyway, it just seems if it is just mirrors you are making and they are going to be ground and then coated, I would think that some of the considerations of the process would not be as important as they would be if you were doing art or something else like that.

Mike


I have successfully experimented with honeycombs in the bottom. However I have come to the conclusion that taking a single piece of glass say 32 in dia by 1.5 or 2 in thick and slump it carefully over a precision ground spherical mold will create a meniscus blank. That blank will still have to be ground like any other telescope mirror however the bulk of the hogging of the curve is eliminated. The bonus to the meniscus idea is that the uniform thickness across the 32 in negates the difference between the COE of plate glass and borosilicate The glass does not have to settle in due to temperature changes in the early evening hours. Plus plate glass is magnitudes cheaper than boro

Bert Weiss
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Re: 2100 degree mold

Postby Bert Weiss » Thu Aug 01, 2013 8:15 am

I would be looking at either Spectrum cullet, or Blenko dalles or cullet. Both of these are inexpensive and the glasses have much better casting characteristics than float glass.

My source for float glass buys a lot of their glass from one factory, however they also buy from others. I usually buy my glass in a 2 ton case, and then all the glass from within that case is compatible. Last year, I got a bunch of free throw away tempred glass from my supplier and luckily, it was all compatible when I tested it with some colored Youghiogheny EZ fuse. Yough is made to be compatible with PPG made in Carlisle PA. My glass was PPG made in Fort Worth TX, but is still compatible. On the other hand, glasses made in different factories are definitely not compatible. BTW, I have been told that 10mm is the cheapest thickness of float glass when you look at the cost per pound.
Bert



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Ron Behrens
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Re: 2100 degree mold

Postby Ron Behrens » Thu Aug 01, 2013 8:21 am

Bert Weiss wrote:I would be looking at either Spectrum cullet, or Blenko dalles or cullet. Both of these are inexpensive and the glasses have much better casting characteristics than float glass.

My source for float glass buys a lot of their glass from one factory, however they also buy from others. I usually buy my glass in a 2 ton case, and then all the glass from within that case is compatible. Last year, I got a bunch of free throw away tempred glass from my supplier and luckily, it was all compatible when I tested it with some colored Youghiogheny EZ fuse. Yough is made to be compatible with PPG made in Carlisle PA. My glass was PPG made in Fort Worth TX, but is still compatible. On the other hand, glasses made in different factories are definitely not compatible. BTW, I have been told that 10mm is the cheapest thickness of float glass when you look at the cost per pound.

Bert
I will look into the cullet. What can one expect to pay for a 2 ton case? Most of my experiments have been with 50 pound glass panes from the same source. We have a recycling business so they become available.
Ron

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Re: 2100 degree mold

Postby Bert Weiss » Thu Aug 01, 2013 8:40 am

Ron Behrens wrote:
Bert Weiss wrote:I would be looking at either Spectrum cullet, or Blenko dalles or cullet. Both of these are inexpensive and the glasses have much better casting characteristics than float glass.

My source for float glass buys a lot of their glass from one factory, however they also buy from others. I usually buy my glass in a 2 ton case, and then all the glass from within that case is compatible. Last year, I got a bunch of free throw away tempred glass from my supplier and luckily, it was all compatible when I tested it with some colored Youghiogheny EZ fuse. Yough is made to be compatible with PPG made in Carlisle PA. My glass was PPG made in Fort Worth TX, but is still compatible. On the other hand, glasses made in different factories are definitely not compatible. BTW, I have been told that 10mm is the cheapest thickness of float glass when you look at the cost per pound.

Bert
I will look into the cullet. What can one expect to pay for a 2 ton case? Most of my experiments have been with 50 pound glass panes from the same source. We have a recycling business so they become available.
Ron
Somewhere plus or minus .60 a pound.
Bert



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Ron Behrens
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Re: 2100 degree mold

Postby Ron Behrens » Fri Aug 02, 2013 6:47 am

Bert Weiss wrote:
Ron Behrens wrote:
Bert Weiss wrote:I would be looking at either Spectrum cullet, or Blenko dalles or cullet. Both of these are inexpensive and the glasses have much better casting characteristics than float glass.

My source for float glass buys a lot of their glass from one factory, however they also buy from others. I usually buy my glass in a 2 ton case, and then all the glass from within that case is compatible. Last year, I got a bunch of free throw away tempred glass from my supplier and luckily, it was all compatible when I tested it with some colored Youghiogheny EZ fuse. Yough is made to be compatible with PPG made in Carlisle PA. My glass was PPG made in Fort Worth TX, but is still compatible. On the other hand, glasses made in different factories are definitely not compatible. BTW, I have been told that 10mm is the cheapest thickness of float glass when you look at the cost per pound.

Bert
I will look into the cullet. What can one expect to pay for a 2 ton case? Most of my experiments have been with 50 pound glass panes from the same source. We have a recycling business so they become available.
Ron
Somewhere plus or minus .60 a pound.


Bert
Just as a an afterthought
Do you have any information on the availability of borosilicate as sooner or later I will want to try that?
Do you have any idea on the cost of borosilicate?
Can you provide any source names or the "spectrum"cullet? I am in central texas
Your help is appreciated
Ron

Bert Weiss
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Re: 2100 degree mold

Postby Bert Weiss » Fri Aug 02, 2013 8:53 am

Ron Behrens wrote:
Bert
Just as a an afterthought
Do you have any information on the availability of borosilicate as sooner or later I will want to try that?
Do you have any idea on the cost of borosilicate?
Can you provide any source names or the "spectrum"cullet? I am in central texas
Your help is appreciated
Ron

Glass company google them. They also sell nuggets designed to be melted in a furnace, at Graingers, I think.
There is a glass on the market called Borofloat. There is also a fairly large industry of lamp workers who use borosilicate glass. There may be cullet or scraps available, I don't know.
Bert



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Ron Behrens
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Re: 2100 degree mold

Postby Ron Behrens » Sat Aug 03, 2013 12:34 pm

Joe Pfeifer wrote:If you go to the website I mentioned, and click on the studio link, it shows other photos. They made a very large boro lens blank. shows the kiln they made it in too. I also know of a guy with a defunct ceramic lab that has some very large very high temp molyD bell style kilns for sintering ceramics. the temps go way higher, like 3000 F! probably get a $150K kiln for 10 cents on the dollar.


Joe
Thanks but I am nowhere ready for that kind of equipment yet
Ron


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